Photonic module and method of manufacture

ABSTRACT

A photonic module, comprising a first waveguide; a second waveguide, disposed on an opposing side of the first waveguide to a substrate; and, a coupling section. One of the first waveguide and the second waveguide is formed of crystalline silicon. The other of the first waveguide and the second waveguide is formed of amorphous silicon. The coupling section is configured to couple light between the first waveguide and the second waveguide. Such a silicon photonic module has enhanced coupling and transmission properties in contrast to conventional modules.

FIELD OF THE INVENTION

The present invention relates to a photonic module and correspondingmethod of manufacture.

BACKGROUND

Conventional photonic modules may include an amorphous silicon waveguidein conjunction with a crystalline silicon component. The amorphoussilicon waveguide is typically fabricated via a three-stage process: (1)plasma enhanced chemical vapour deposition, PECVD, of amorphous siliconin a cavity, where the cavity is formed by performing an etch on asilicon-on-insulator substrate, SOI; (2) chemical mechanical polishing,CMP, to obtain a uniform topography and a height equal to that of theSOI; and, (3) lithography and etching to produce an amorphous siliconstrip waveguide.

However, such photonic modules, and the corresponding method ofmanufacture, have a number of disadvantages. Firstly, PECVD in a cavityresults in two different growth-fronts; vertical, starting from the bedof the cavity, and horizontal, starting from the sidewalls of thecavity. These vertical and horizontal growth-fronts meet along adiagonal surface, which creates voids. Such voids are particularlyundesirable because they act as scattering centres, which leads tosignificant optical insertion losses. Voids are also produced at theamorphous-crystalline interface.

Further, CMP is a challenging process, the control of which affectsoverall optical performance. In particular, non-ideal CMP, in which toomuch or too little amorphous silicon is removed, results in a steptopology at the amorphous-crystalline interface instead of a flatsurface. This leads to insertion loss and undesirable higher-order mode,HOM, excitation. Non-ideal CMP also causes dishing, i.e., the thicknessof the amorphous silicon waveguide varies continuously along thepropagation direction, where the amorphous silicon is thinnest at thecentre of the cavity and thickest at the interface. Dishing leads tobirefringence which is undesirable because the optical performance willthen be polarisation dependent.

There is therefore a need for an improved photonic module andcorresponding method of manufacture.

SUMMARY

Accordingly, in a first aspect, embodiments of the present inventionprovide a photonic module comprising: a first waveguide; a secondwaveguide, disposed on an opposing side of the first waveguide to asubstrate; and, a coupling section; wherein one of the first waveguideand the second waveguide is formed of crystalline silicon; the other ofthe first waveguide and the second waveguide is formed of amorphoussilicon; and, the coupling section is configured to couple light betweenthe first waveguide and the second waveguide.

Such a silicon photonic module has enhanced coupling and transmissionproperties in contrast to conventional modules. Notably, the photonicmodule provides an escalator scheme which can vertically couple light upinto the second waveguide. Moreover, specific optical modes can becoupled across a broad wavelength range.

The photonic module may have any one, or any combination insofar as theyare compatible, of the following optional features.

The photonic module can be implemented in both large waveguide and smallwaveguide platforms. For example, the photonic module can be implementedin a 3 μm waveguide platform, or a 0.3 μm waveguide platform.

The substrate may define a horizontal plane, the first waveguide may bepositioned above the substrate, and the second waveguide may bepositioned above the first waveguide. In this way, the first waveguideis located between the second waveguide and the substrate.

The coupling section may comprise a tapered portion of at least one ofthe first waveguide and the second waveguide. The tapered portioncorresponds to a reduction in width from a first value to a second valuein a direction parallel or substantially parallel to the respectivewaveguide, wherein the width is a transverse width. In this way, byspatially compressing the optical modes propagating through the at leastone of the waveguides, the tapered portion induces an efficient transferof optical power between the waveguides. Accordingly, the taperedportion facilitates efficient evanescent coupling. The thickness andwidth of the second waveguide may be chosen so that a phase-matchingpoint exists along the coupling section.

The coupling section may comprise a tapered portion of the firstwaveguide, tapering from a first width to a second width along a firstdirection, and a tapered portion of the second waveguide, tapering froma first width to a second width along a second direction; wherein thefirst direction and second direction are antiparallel or substantiallyantiparallel. By substantially antiparallel, it may be meant that anangle between vectors describing the two directions may be 180°±0.5°,±1°, ±2°, or ±5°. In this way, a narrowed portion, having the secondwidth, of the first waveguide is aligned with a non-tapered portion ofthe second waveguide (i.e. a portion having the first width), and anarrowed portion, having the second width, of the second waveguide isaligned with a non-tapered portion of the first waveguide (i.e. aportion having the first width). Accordingly, the coupling section isconfigured to induce an efficient transfer of optical power from thefirst waveguide to the second waveguide, and vice versa. The photonicmodule is bidirectional.

The first width along the first direction, may be equal to, or differentto, the first width along the second direction. Similarly, the secondwidth along the first direction, may be equal to, or different to, thesecond width along the second direction.

The photonic module may further comprise a first cladding disposed so asto at least partially surround the first waveguide and a second claddingdisposed so as to at least partially surround the second waveguide. Inthis way, the photonic module has improved efficiency due to a reductionin optical losses. Preferably, the first cladding and the secondcladding are formed of silicon dioxide, which has a lower refractiveindex than the silicon waveguide components; accordingly, opticalleakage is reduced.

A length of the coupling section may be greater than a maximumtransverse width of the first waveguide and a maximum transverse widthof the second waveguide. The length of the coupling section may be morethan 3 μm, for example at least 4 μm. The length of the coupling sectionmay be no more than 10 μm. The maximum transverse widths may be at least2 μm. The maximum transverse widths may be no more than 4 μm. In thisway, the coupling section is adiabatic.

In other examples, the length of the coupling section may be the same asor smaller than a maximum transverse width of the first waveguide and amaximum transverse width of the second waveguide.

The photonic module may further comprise an intermediary layer disposedin-between the first waveguide and the second waveguide. In this way,the second waveguide is disposed on the intermediary layer. Theintermediary layer may be formed of silicon oxide. The intermediarylayer may be at most 100 nm tall (i.e. as measured from the top of thefirst waveguide to the bottom of the second waveguide).

The first waveguide may have a port or facet, through which light isreceived or transmitted, which is around 3 μm wide. The second waveguidemay have a port or facet, through which light is received ortransmitted, which is around 3 μm wide. The port or facet in the firstwaveguide may be an input port, in that it is configured to receivelight and transmit it to the coupling region. The port or facet in thesecond waveguide may be an output port, in that it is configured totransmit light received from the coupling region out of the photonicmodule.

The substrate may comprise a buried oxide, BOX, layer. The substrate mayfurther comprise a silicon substrate layer, located on an opposing sideof the buried oxide layer to the first waveguide.

The first waveguide may be formed of crystalline silicon and the secondwaveguide may be formed of amorphous silicon. Alternatively, the firstwaveguide may be formed of amorphous silicon and the second waveguidemay be formed of crystalline silicon.

The photonic module may further comprise an additional silicon dioxidecladding layer disposed on an opposing side of the second waveguide tothe first waveguide. In this way, optical leakage is further reduced.

The photonic module may further comprise a third waveguide and a secondcoupling section, wherein the second coupling section is configured tocouple light between the second waveguide and the third waveguide. Thesecond waveguide may be disposed on an opposing side of the thirdwaveguide to the substrate.

The third waveguide may be positioned on the same horizontal plane asthe first waveguide. Alternatively, the third waveguide may be disposedon an opposing side of the second waveguide to the first waveguide. Thefirst waveguide and the third waveguide may be formed of crystallinesilicon, and the second waveguide may be formed of amorphous silicon. Inthis way, the photonic module may couple light from crystalline siliconinto amorphous silicon, and back into crystalline silicon.Alternatively, the first waveguide and the third waveguide may be formedof amorphous silicon, and the second waveguide may be formed ofcrystalline silicon. In this way, the photonic module may couple lightfrom amorphous silicon into crystalline silicon, and back into amorphoussilicon.

The third waveguide may include any one, or any combination insofar asthey are compatible, of the optional features of the first waveguide.

The second coupling section may include any one, or any combinationinsofar as they are compatible, of the optional features of the firstcoupling section.

In a second aspect, embodiments of the invention provide a componentincluding the photonic module of the first aspect, wherein the componentincludes one or more crossing waveguides, a portion of the or eachcrossing waveguides being located between the second waveguide and thesubstrate; and the crossing waveguides are optically insulated from thewaveguides of the photonic module. In this way, the componentfacilitates simultaneous power transmission via the photonic module, aswell as via the crossing waveguides. Accordingly, multi-directionalpower transmission is facilitated; for example, the one or more crossingwaveguides may be angled relative to the photonic module.

The photonic module as used in the second aspect may include any one, orany combination insofar as they are compatible, of the optional featuresof the photonic module of the first aspect.

In a third aspect, embodiments of the invention provide a Mach-ZenderInterferometer, MZI, having two arms, at least one of the armscomprising at least one photonic module of the first aspect. Thephotonic module as used in the third aspect may include any one, or anycombination insofar as they are compatible, of the optional features ofthe photonic module of the first aspect.

In a fourth aspect, embodiments of the invention provide an arrayedwaveguide grating, AWG, the AWG including a plurality of output or inputwaveguides, at least one of the output or input waveguides comprising aphotonic module of the first aspect. The photonic module of the fourthaspect may include any one, or any combination insofar as they arecompatible, of the optional features of the photonic module of the firstaspect.

Accordingly, the photonic module introduces a path-length imbalancebetween the two arms of the MZI, or between the plurality of output orinput waveguides of the AWG. Further, the MZI or AWG may couple lightfrom crystalline silicon to amorphous silicon and back into crystallinesilicon, or, couple light from amorphous silicon to crystalline siliconand back into amorphous silicon.

According to a fifth aspect, embodiments of the invention provide amethod of fabricating a photonic module on a substrate, the methodincluding: performing a first etching process on a first layer to form afirst waveguide; depositing a second layer on an opposing side of thefirst waveguide to a substrate; and performing a second etching processon the second layer to form a second waveguide; wherein: the steps ofperforming the first etching process and/or the second etching processalso form a coupling section for coupling light between the firstwaveguide and the second waveguide; one of the first layer and thesecond layer is formed of crystalline silicon; and the other of thefirst layer and the second layer is formed of amorphous silicon.

The method advantageously results in a photonic module having enhancedcoupling and transmission properties.

The method may include a step, performed before etching the first layer,of depositing the first layer on the substrate. The method may beperformed on a silicon-on-insulator wafer, wherein the first layer isprovided by the silicon device or silicon-on-insulator layer.

The step of performing the first etching process may also form a taperedportion of the first waveguide, the coupling section comprising thetapered portion of the first waveguide.

The step of performing the second etching process may also form atapered portion of the second waveguide, the coupling section comprisingthe tapered portion of the second waveguide.

The step of performing the first etching process may also form a taperedportion of the first waveguide, the tapered portion tapering from afirst width to a second width along a first direction and the couplingsection comprising the tapered portion of the first waveguide; and thestep of performing the second etching process may also form a taperedportion of the second waveguide, the tapered portion tapering from afirst width to a second width along a second direction and the couplingsection further comprising the tapered portion of the second waveguide;such that the first direction and second direction are antiparallel orsubstantially antiparallel. By substantially antiparallel, it may bemeant that an angle between vectors describing the two directions may be180°±0.5°, ±1°, ±2°, or ±5°.

In this way, no cavity etch or CMP is required. The second waveguide maybe blanket-deposited on the opposing side of the first waveguide to thesubstrate, instead of over a cavity. Since there is no CMP,CMP-associated problems are also eliminated. Also since multiplegrowth-fronts are eliminated, voids or seams detrimental to opticalperformance are avoided.

One or more etches within the first etching process and/or the secondetching process may be performed anisotropically.

The first etching process may include an initial step of applying afirst mask to the first layer; and the second etching process mayinclude an initial step of applying a second mask to the second layer.

The method further includes depositing a first cladding so as to atleast partially surround the first waveguide; and depositing a secondcladding so as to at least partially surround the second waveguide. Inthis way, the photonic module has improved efficiency due to a reductionin optical loss. The first cladding and the second cladding may beformed of silicon dioxide. The silicon dioxide has a lower refractiveindex than the silicon waveguide components. Accordingly, opticalleakage is reduced.

The step of depositing the second layer may further include an initialstep of depositing an intermediary layer on an opposing side of thefirst waveguide to the substrate. The intermediary layer may be formedof silicon oxide. In this way, the second layer is blanket-deposited onthe intermediary layer of silicon oxide, instead of a cavity.Accordingly, void formation is avoided.

The substrate may comprise a buried oxide, BOX, layer. The substrate mayfurther comprise a silicon substrate layer, located on an opposing sideof the buried oxide layer to the first waveguide.

The first waveguide may be formed of crystalline silicon and the secondwaveguide may be formed of amorphous silicon.

Alternatively, the first waveguide may be formed of amorphous siliconand the second waveguide may be formed of crystalline silicon.

The method may further include depositing an additional silicon dioxidecladding layer on an opposing side of the second waveguide to the firstwaveguide.

In a sixth aspect, embodiments of the present invention provide awaveguide structure, comprising:

-   -   a first waveguide on a substrate; and    -   a second waveguide on the substrate,    -   the first waveguide and the second waveguide each being        substantially parallel to the substrate,    -   the first waveguide being at a different height than the second        waveguide, and    -   the waveguides being configured to cause light propagating in        the first waveguide to couple into the second waveguide.

The waveguide structure of the sixth aspect, including at least thefirst waveguide, second waveguide, and substrate, may have any one, orany combination insofar as they are compatible, of the optional featuresas set out with reference to the first aspect.

Further aspects of the present invention provide: a computer programcomprising code which, when run on a computer, causes the computer toperform the method of the fifth aspect; a computer readable mediumstoring a computer program comprising code which, when run on acomputer, causes the computer to perform the method of the fifth aspect;and a computer system programmed to perform the method of the fifthaspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a top-down schematic view of a photonic module inaccordance with an embodiment of the present invention;

FIG. 2A illustrates a cross-sectional view of the photonic module ofFIG. 1 ;

FIG. 2B illustrates a cross-sectional view of the photonic module ofFIG. 1 ;

FIG. 3A illustrates a component in accordance with an embodiment of thepresent invention;

FIG. 3B illustrates an alternative configuration of the component ofFIG. 3A;

FIG. 3C illustrates yet another configuration of the component of FIG.3A;

FIG. 3D illustrates a further configuration of the component of FIG. 3A;

FIG. 4 illustrates an MZI in accordance with an embodiment of thepresent invention;

FIG. 5 illustrates an AWG in accordance with an embodiment of thepresent invention;

FIG. 6A illustrates a cross-sectional view of the photonic module ofFIG. 1 ;

FIG. 6B illustrates a cross-sectional view of the photonic module ofFIG. 1 ;

FIG. 6C illustrates a cross-sectional view of the photonic module ofFIG. 1 ;

FIG. 7A depicts simulation results of the photonic module at thecross-section as depicted in FIG. 6A;

FIG. 7B depicts simulation results of the photonic module at thecross-section as depicted in FIG. 6B; and

FIG. 7C depicts simulation results of the photonic module at thecross-section as depicted in FIG. 6C.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

Aspects and embodiments of the present invention will now be discussedwith reference to the accompanying figures. Further aspects andembodiments will be apparent to those skilled in the art.

FIG. 1 illustrates a top-down schematic view of a photonic module 1. Thephotonic module 1 includes a substrate 5, on which a first waveguide 10is disposed. A second waveguide 20 is disposed on an opposing side ofthe first waveguide 10 to the substrate 5, i.e., above the firstwaveguide 10 and the substrate 5 in the ‘z’ direction. The photonicmodule 1 also includes a coupling section 25 for coupling light betweenthe first waveguide 10 and the second waveguide 20. The coupling section25 comprises a tapered portion 30 of the first waveguide, tapering froma first width to a second width along a first direction, and a taperedportion 35 of the second waveguide, tapering from a first width to asecond width along a second direction. The tapered portion 30 extendsbetween a non-tapered portion 29, having the first width, and a narrowedportion 31 of the first waveguide 10, having the second width.Similarly, the tapered portion 35 extends between a non-tapered portion34, having the first width, and a narrowed portion 36 of the secondwaveguide 20, having the second width. In this example, the firstdirection and the second direction are antiparallel. Alternatively, thefirst direction and the second direction may be substantiallyantiparallel. By substantially antiparallel, it may be meant that anangle between vectors describing the two directions may be 180°±0.5°,±1°, ±2°, or ±5°. As such, the narrowed portion 31 of the firstwaveguide is aligned with the non-tapered portion 34 of the secondwaveguide, and the narrowed portion 36 of the second waveguide isaligned with the non-tapered portion 29 of the first waveguide. Thephotonic module 1 is bidirectional; the coupling section 25 induces anefficient transfer of optical power from the first waveguide 10 to thesecond waveguide 20, and vice versa.

As shown in FIG. 1 , the first width along the first direction is equalto the first width along the second direction. That is to say, the firstand second waveguide have the same first width and same second width. Inan alternative, not shown, the first and second waveguides haverespectively different first widths and/or respectively different secondwidths.

The length of the coupling section 25 is greater than a maximumtransverse width of the first waveguide 10 and a maximum transversewidth of the second waveguide 20. Accordingly, the length of thecoupling section 25 is greater than the transverse widths of thenon-tapered portion 29 of the first waveguide 10 and the non-taperedportion 34 of the second waveguide 20. The length of the couplingsection may be at least 4 μm but no more than 10 μm, and the maximumtransverse widths may be at least 2 μm but no more than 4 μm.Accordingly, the photonic module is dimensioned such that the couplingsection is adiabatic.

FIG. 2A illustrates a cross-sectional view of the photonic module 1,along the line A-A′ in FIG. 1 wherein the cross-section intersects thenon-tapered portion 29 of the first waveguide and the narrowed portion36 of the second waveguide. Similarly, FIG. 2B illustrates across-sectional view of the photonic module 1, along the line B-B′ inFIG. 1 wherein the cross-section intersects the narrowed portion 31 ofthe first waveguide and the non-tapered portion 34 of the secondwaveguide.

As depicted in FIGS. 2A and 2B, a first cladding 12, of equal height tothe first waveguide 10, is disposed adjacent to the first waveguide 10,so as to at least partially surround the first waveguide 10. A secondcladding 22, of equal height to the second waveguide 20, is disposedadjacent to the second waveguide 20, so as to at least partiallysurround the second waveguide 20. The first cladding 12 and the secondcladding 22 are formed of silicon dioxide for reducing optical leakage.An additional silicon dioxide cladding layer 38 is disposed on the uppersurfaces of the second waveguide 20 and the second cladding 22. In thisexample, the first waveguide is a rib waveguide in that the optical modeis chiefly confined to the upstanding rib portion 10 and does not leakinto the silicon device layer 8. In an alternative example, the firstwaveguide is a ridge waveguide, and the optical mode is contained inboth an upstanding ridge portion and a corresponding slab portion of thewaveguide.

An intermediary layer 15 is disposed in between the first waveguide 10and the second waveguide 20. The intermediary layer 15 also separatesthe first cladding 12 and the second cladding 22. The intermediary layer15 is of uniform thickness and is bound between a first planar surface,defined by the first waveguide 10 and the first cladding 12, and asecond planar surface, defined by the second waveguide 20 and the secondcladding 22. The intermediary layer 15 is formed of silicon oxide. Inthis example, the intermediary layer 15 is at most 100 nm tall (i.e. asmeasured from the top of the first waveguide 10 to the bottom of thesecond waveguide 20).

The substrate includes a buried oxide, BOX, layer 7, on which a silicondevice layer 8 is disposed. In turn, the first waveguide is disposed onthe silicon device layer 8. The first waveguide 10 is formed ofcrystalline silicon, and the second waveguide 20 is formed of amorphoussilicon. Below the buried oxide layer there may be a silicon substratelayer.

FIG. 3A illustrates a component 40 comprising a photonic module 1 havinga third waveguide 42 and a second coupling section 43. The secondcoupling section 43 is configured to couple light between the secondwaveguide 20 and the third waveguide 42. The second coupling section 43is a mirror image of the first coupling section 25 reflected about thez-y plane.

The second waveguide 20 is disposed on an opposing side of the thirdwaveguide 42 to the substrate 5. The third waveguide 42 is positioned onthe same horizontal plane as the first waveguide 10. Alternatively, thethird waveguide 42 may be disposed on an opposing side of the secondwaveguide 20 to the first waveguide 10. The component 40 includes acrossing waveguide 45, a portion of which is located between the secondwaveguide 20 and the substrate 5. The crossing waveguide 45 is opticallyinsulated from the first waveguide 10, the second waveguide 20 and thethird waveguide 42 of the photonic module 1. The second waveguide 20forms a bridge-like structure over the crossing waveguide 45. Thecrossing waveguide 45 is straight and extends perpendicular to alongitudinal axis of the photonic module 1. The component may form aportion of a photonic integrated circuit (PIC).

FIG. 3B illustrates an alternative example of the component 40, whereinthe crossing waveguide 45 is angled relative to the longitudinal axis ofthe photonic module 1. Like features are indicated by like referencenumerals.

FIG. 3C illustrates yet another example of the component 40, wherein thecrossing waveguide 45 intersects the longitudinal axis of the photonicmodule 1 at a non-zero angle, but the crossing waveguide is curved suchthat a first end 46 and a second end 47 extend parallel to thelongitudinal axis of the photonic module 1. Like features are indicatedby like reference numerals.

FIG. 3 d illustrates a further example of the component 40, wherein thecomponent includes more than one crossing waveguide 45. Like featuresare indicated by like reference numerals.

FIG. 4 illustrates a Mach-Zehnder interferometer (MZI) 50 having twoarms 55. One of the arms 55 includes a photonic module 1 to introduce apath-length imbalance between the two arms 55. The photonic module 1facilitates a thermal performance of the MZI 50.

FIG. 5 illustrates an arrayed waveguide grating (AWG) 60 having multipleoutput/input waveguides 65. Each of the output/input waveguides includesa photonic module 1 to introduce a path-length imbalance between theoutput/input waveguides 65. The AWG 60 facilitates transmission in afirst direction via the photonic modules 1, and transmission in a seconddirection via the output/input waveguides 65, which may be regularsilicon waveguides. The first and second directions are perpendicular orsubstantially perpendicular. The photonic module 1 facilitates a thermalperformance of the AWG 60.

FIG. 6A depicts an input port of the first waveguide 10, through whichlight is received. The input port is around 3 μm wide and is configuredto receive light and transmit it to the coupling section 25. In anotherexample, the first waveguide 10 has an output port, through which lightis transmitted, which is around 3 μm wide and is configured to transmitlight received from the coupling section out of the photonic module.

The thickness and width of the second waveguide 20 is chosen so that aphase matching point exists along the coupling section 25. FIG. 6Bdepicts a cross-sectional view of a phase matching point.

FIG. 6C depicts an output port of the second waveguide 20, through whichlight is transmitted. The output port is around 3 μm wide and isconfigured to transmit light received from the coupling region out ofthe photonic module. In another example, the second waveguide 20 has aninput port, through which light is received, which is around 3 μm wideand is configured to receive light and transmit it to the couplingregion 25.

FIG. 7A depicts the optical mode at the input port of FIG. 6A; theoptical mode is concentrated in the region corresponding to the firstwaveguide 10. FIG. 7C depicts the optical mode at the output port of 7Cafter it has passed through the coupling region; the optical mode isconcentrated in the region corresponding to the second waveguide 20.

FIG. 7B depicts the optical mode at the phase matching point of FIG. 6B.

The photonic module 1 is fabricated either by depositing a first layeron the substrate 5, the substrate 5 includes a buried oxide, BOX, layer7 or by providing a silicon-on-insulator wafer, the silicon device layerof the wafer providing the first layer. The first layer may be formed ofeither crystalline silicon (e.g. in the case of an SOI wafer) oramorphous silicon (e.g. in the case of depositing a first layer). Next,a first mask is applied to the first layer. This can be performed viaphotolithography. The first mask is either dimensioned or etched so asto cover a portion of the first layer corresponding to the desired shapeof the first waveguide 10. A first etch is then performed to produce thefirst waveguide 10, which includes a tapered portion 30. The taperedportion 30 extends between a non-tapered portion 29 and a narrowedportion 31 of the first waveguide 10. An anisotropic etch may be usedwhen performing the first etch.

A first cladding 12 is deposited (for example by chemical vapourdeposition) adjacent to the first waveguide 10 to the same height as thefirst waveguide 10, such that the upper surfaces of the first cladding12 and the first waveguide 10 form a planar surface. An intermediarylayer 15 is then deposited on the planar surface formed by the uppersurfaces of the first cladding 12 and the first waveguide 10. Theintermediary layer 15 is formed of silicon oxide. The intermediary layer15 is at most 100 nm tall (i.e. as measured from the top of the firstwaveguide 10 to the bottom of the second waveguide 20).

Next, a second layer is deposited on the intermediary layer 15, and asecond mask is applied to the second layer. If the first layer is formedof crystalline silicon, then the second layer is formed of amorphoussilicon, and vice versa. The second mask is dimensioned so as to cover aportion of the second layer corresponding to the desired shape of thesecond waveguide 20. A second etch is then performed to produce thesecond waveguide 20, which includes a tapered portion 35. The taperedportion 35 extends between a non-tapered portion 34 and a narrowedportion 36 of the second waveguide 20. The transverse width of thetapered portion 30 decreases in a first direction along a length of thephotonic module 1, whereas the transverse width of the tapered portion35 decreases in an opposite direction to that of the first direction. Assuch, the narrowed portion 31 of the first waveguide is aligned with thenon-tapered portion 34 of the second waveguide, and the narrowed portion36 of the second waveguide is aligned with the non-tapered portion 29 ofthe first waveguide. Accordingly, the coupling section 25 comprises thetapered portion 30 of the first waveguide and the tapered portion 35 ofthe second waveguide. An anisotropic etch may be used when performingthe second etch.

A second cladding 22 is deposited adjacent to the second waveguide 20 tothe same height as the second waveguide 20, such that the upper surfacesof the second cladding 22 and second waveguide 20 form a planar surface.An additional silicon dioxide cladding layer 28 is deposited on theplanar surface formed by the upper surfaces of the second cladding 22and the second waveguide 20.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

LIST OF FEATURES

-   1: Photonic module-   5: Substrate-   7: Buried oxide, BOX, layer-   8: Silicon device layer-   10: First waveguide-   12: First cladding-   15: Intermediary layer-   20: Second waveguide-   22: Second cladding-   25: Coupling section-   29: Non-tapered portion of the first waveguide-   30: Tapered portion of the first waveguide-   31: Narrowed portion of the first waveguide-   34: Non-tapered portion of the second waveguide-   35: Tapered portion of the second waveguide-   36: Narrowed portion of the second waveguide-   38: Additional silicon dioxide cladding-   40: Component-   42: Third waveguide-   43: Second coupling section-   45: Crossing waveguide-   46: First end of crossing waveguide-   47: Second end of crossing waveguide-   50: MZI-   55: MZI arm-   60: AWG-   65: AWG output/input waveguide

1. A photonic module, comprising: a first waveguide; a second waveguide,disposed on an opposing side of the first waveguide to a substrate; and,a coupling section; wherein one of the first waveguide and the secondwaveguide is formed of crystalline silicon; the other of the firstwaveguide and the second waveguide is formed of amorphous silicon; and,the coupling section is configured to couple light between the firstwaveguide and the second waveguide.
 2. The photonic module of claim 1,wherein the substrate defines a horizontal plane, the first waveguide ispositioned above the substrate, and the second waveguide is positionedabove the first waveguide.
 3. The photonic module of claim 1, whereinthe coupling section comprises a tapered portion of at least one of thefirst waveguide and the second waveguide.
 4. The photonic module ofclaim 1, wherein the coupling section comprises: a tapered portion ofthe first waveguide, tapering from a first width to a second width alonga first direction; and a tapered portion of the second waveguide,tapering from a first width to a second width along a second direction;wherein the first direction and second direction are antiparallel. 5.The photonic module of claim 1, further comprising a first claddingdisposed so as to at least partially surround the first waveguide and asecond cladding disposed so as to at least partially surround the secondwaveguide.
 6. The photonic module of claim 5, wherein the first claddingand the second cladding are formed of silicon dioxide.
 7. The photonicmodule of claim 1, wherein a length of the coupling section is greaterthan: a maximum transverse width of the first waveguide; and a maximumtransverse width of the second waveguide.
 8. The photonic module ofclaim 7, wherein the length of the coupling section is at least 4 μm butno more than 10 μm, and the maximum transverse widths are at least 2 μmbut not more than 4 μm.
 9. The photonic module of claim 1, furthercomprising an intermediary layer disposed in-between the first waveguideand the second waveguide.
 10. The photonic module of claim 9, whereinthe intermediary layer is formed of silicon oxide.
 11. The photonicmodule of claim 1, wherein the substrate comprises a buried oxide, BOX,layer.
 12. The photonic module of claim 1, wherein the first waveguideis formed of crystalline silicon and the second waveguide is formed ofamorphous silicon.
 13. The photonic module of claim 1, wherein the firstwaveguide is formed of amorphous silicon and the second waveguide isformed of crystalline silicon.
 14. The photonic module of claim 1,further comprising a third waveguide and a second coupling section,wherein the second coupling section is configured to couple lightbetween the second waveguide and the third waveguide, and wherein thesecond waveguide is disposed on an opposing side of the third waveguideto the substrate.
 15. A component comprising the photonic module ofclaim 1, wherein: the component includes one or more crossingwaveguides, a portion of the or each crossing waveguides being locatedbetween the second waveguide and the substrate; and the crossingwaveguides are optically insulated from the waveguides of the photonicmodule.
 16. (canceled)
 17. (canceled)
 18. A method of fabricating aphotonic module on a substrate, the method including: performing a firstetching process on a first layer to form a first waveguide; depositing asecond layer on an opposing side of the first waveguide to a substrate;and performing a second etching process on the second layer to form asecond waveguide; wherein: the steps of performing the first etchingprocess and/or the second etching process also form a coupling sectionfor coupling light between the first waveguide and the second waveguide;one of the first layer and the second layer is formed of crystallinesilicon; and the other of the first layer and the second layer is formedof amorphous silicon.
 19. The method of claim 18, wherein the step ofperforming the first etching process forms a tapered portion of thefirst waveguide, the coupling section comprising the tapered portion ofthe first waveguide.
 20. The method of claim 18, wherein the step ofperforming the second etching process forms a tapered portion of thesecond waveguide, the coupling section comprising the tapered portion ofthe second waveguide. 21.-24. (canceled)